The battery system for use in vehicles is generally a double-redundancy system in consideration of abnormality that may develop while the system is operating. The double-redundancy system comprises a main battery unit and an auxiliary battery unit totally identical to the main battery unit. In operation, the battery system fails to output the rated power even if only one of the battery modules constituting the main battery unit enters an abnormal state. In this case, the main battery unit is disconnected from the output system, and the auxiliary battery unit is connected to the output system and used in place of the main battery unit. Inevitably, twice as many battery modules must be mounted on the vehicle in order to provide the rated power.
The battery system, particularly one mounted on a special vehicle, such as a submersible vehicle and a flight vehicle, should keep operating once it has been started. Therefore, the battery modules cannot be replaced by other modules. Thus, each battery module must have high reliability. The reliability of the battery module is, however, technically limited.